Attenuated Total Reflection-Scanning Near-Field Raman Spectroscopy

Abstract

A facility for scanning near-field Raman spectroscopy (SNRS) using the attenuated total reflection (ATR) method was built to obtain topographic and Raman images with a spatial resolution higher than the diffraction limit. A Raman spectrum of copper phthalocyanine (CuPc) was obtained with an accumulation time of 1000 s without using surface plasmon excitation or the electronic resonance effect of the molecules. The optimum conditions for enhancing the Raman signal using localized surface plasmon (LSP) on Ag island films were found to be 5 nm in thickness and an incident angle of 50°. Under these conditions, the Raman signal from CuPc (1.5 nm) was enhanced by a factor of ca. 300, which gives 36 counts/s for ca. 30,000 molecules, and enables us to obtain the Raman spectra in 2 s. Interestingly, the scanning near-field optical microscopy (SNOM)-Raman intensity increases with decreasing thickness of CuPc samples below 3 nm due to the near-field effect between the fiber probe and Ag films, while it remains constant for 5–30 nm in CuPc thickness. The polarization dependence of the signal intensity supports this interpretation. In contrast, without the Ag underlayer, the SNOM-Raman scattering intensity is not prominently enhanced due to the weaker interaction between the dielectric sample and the probe. In both cases, the SNOM-Raman scattering signal arises within ca. 5 nm from the sample surface.